Tuesday, June 20, 2017

Welcome back to this 20–part series exploring the equine head. In Part 1 we introduced the basic evolutionary basis for its structure and now we’ll dive into structural details to better flesh out our understanding. The equine head is truly a marvel of bioengineering and to understand it is to literally understand what it means to be a horse. Fantastically, everything about the horse’s biology is embodied in his head, making its accurate portrayal in sculpture all the more critical for authenticity. That's to say, to sculpt equine realism doesn't mean to simply sculpt what we see—it means to sculpt our subject's biology in the full breadth of what that means. We're literally sculpting his evolutionary history as expressed by his unique physique. If we don't understand the hows and whys of that evolutionary history then, we'll not only miss out on a huge chunk of what it means to be this animal, but we'll risk making errors due to oversight or misunderstanding. So let’s continue our journey of discovery, starting with his gut, the feature that helps to define this curious animal…

Digestion

As we learned in Part I, as Hyracotherium ventured out onto the newly forming grassy plains, his body had to undergo a series of specific adaptations. One of these key adaptations was in the viscera—and yes—this has everything to do with his head, so hang tight…

In order to eat grass, the animal needed a digestive system that could process grasses rather than forest vegetation. Yet this is no easy task! Grass is a comparatively poor food source that possesses cellulose, the complex sugar found in fibrous plants. Grasses cannot be broken down by mammals without the aid of gut bacteria to break up this cellulose into volatile fatty acids the animal’s body can process. Yet this is a time–consuming method of digestion that requires a chamber for this organic matter to be stored to allow the symbiotic bacteria to work their magic.

There are two different ways this relationship has been expressed in ungulates. One is ruminant digestion (or pre–gastric fermentation) and the other is cecal digestion (or post–gastric fermentation). Artiodactyls (even–toed animals) evolved ruminant digestion first, developing four stomach chambers. The first two chambers, the rumen and reticulum are where bacterial fermentation occurs. Then the food material is regurgitated and chewed again, known as “chewing the cud.” Upon being swallowed for a second time, the food matter passes through a special opening into the last two chambers of the stomach, the omasum and abomasums, where further digestion takes place. Pound per pound, rumination is the most efficient means for extracting nutrients from grasses largely because food matter ferments for the longest period of time. It takes about 70–90 hours for food to pass through a cow, for example.

In contrast, Perissodactyls (odd–toed animals) evolved cecal digestion, or digestion in the cecum (also called the “hind gut” or “water gut,” equivalent to the human appendix) which grew to enormous size to provide a fermentation chamber. An adult horse of average weight will have a cecum of about 3–4 feet long (91–122cm) and with a capacity of about 7–8 gallons (26–30l). In comparison, the stomach of an adult horse takes up only about 10% of the digestive track while the cecum (and colon) comprises about 60% (and the small intestine about 30%). In this scenario, food is chewed and swallowed into a comparatively small stomach in which the foodstuffs are turned into a slurry with digestive juices and then passed into the cecum (via the small intestine) for bacterial processing. From the cecum, the materials then travel to the colons for further digestion. Because cecal digestion is a more effective digestive system in animals under 11 pounds, it’s believed Perissodactyla may have adopted it while still small during the Paleocene. However, cecal digestion extracts about 30% less energy from the same food stuff as does ruminant digestion. Nonetheless, the benefit is that it takes far less time to digest the food matter—only about 48 hours.

Consequently, this difference allows equines to flourish in ecological niches where few other animals could survive, particularly ruminants. Biological data reveals that wild equines usually target the worst, lowest quality and highest fiber roughage they encounter. For example, ruminates may eat only the leaves, but equines will eat the stems left behind (as observed between Plains zebras and Gnus). In North America, feral horses and cattle may compete for the same forage, but horses utilize territories not exploited by cattle such as those far away from water sources (especially in winter) and at higher elevations. Likewise, feral horses in North America don’t compete with the Pronghorn, a native ungulate, since this animal eats mostly shrubs, herbs, and woody forage. In fact, many wild equines thrive in areas characterized by such poor quality vegetation that ruminants appear to avoid the area altogether! For instance, wild asses and the Takh thrive in regions other large grazers deliberately avoid. Further studies support this observation by showing that unless a certain level of fiber is provided by the habitat, a ruminant will starve and so will avoid that habitat.

So, what’s the reason? Time! A ruminant’s system can only process a limited amount of food in a fixed period, which happens to be rather long. Conversely, an equine’s response to poor vegetation is to simply eat more since his form of digestion can process nutrients far faster. So while a ruminant digestion is more efficient per unit of energy per unit of time, a horse can extract far more energy from a grassy habitat than a ruminant. There’s a very important reason why horses love to eat!

Moreover, unlike ruminants, the horse’s digestive system allows him to “eat on the go,” one moment munching lunch while the next avoiding becoming one. He was the original “fast food junkie”! This digestive design is also ideal for migration because he doesn’t need to be stationary to chew his cud. So while cecal digestion is considered comparatively archaic, it actually panned out as an evolutionary advantage for Equus.

However, all this extra time a horse spends grazing amounts to about 15–20 hours a day, a bit longer than the 8–10 hours a day a ruminant spends. All this “head–down time” creates a greater vulnerability to predation, which has some interesting implications for how his head developed, which we’ll get to a little bit later…anyway, for now…

Teeth

Yet the equine required more than a new digestive system to deal with abrasive grasses—he had to have teeth that could chew it while also coping with the gritty soil it grew in, and all without being milled down to nubs in short order. Unlike fruits, shoots, sprouts, and leaves, which either depend on being eaten to propagate or can suffer loss of some material without dying, the blades of grass are the plant itself—and plants enduring heavy herbivore depletion usually evolve various defensive strategies. With grasses, this involved the infusion of sturdy silica particles (or phytoliths) into the cell walls, essentially locking the nutrients in a silica skeleton. Similar to glass powder and working like sandpaper, it’s these silica particles that lend shape and stiffness to a blade of grass, even when dead. So to pulverize this silica skeleton to extract the nutrients—to the necessary consistency of fine cornmeal—a horse has to do a lot of chewing, something which would rapidly wear down his teeth due to those abrasive silica particles.

In response, the horse lost his short, gently–cusped, bunodont teeth for a design that could withstand this constant abrasive onslaught. As such, the bunodont dentin cusps became elongated into long prongs sheathed in hard enamel which then formed into slicing ridges. Hard cementum (once vestigial in the old bunodont teeth) began to encapsulate the entire tooth and fill in any spaces, particularly between the dentin prongs. Eventually, the equine developed continually erupting (hypselodont), long–crowned teeth (hysodont teeth) with long ridges made of blended dentin cusps (lophodont teeth) coated in tough enamel and encapsulated with hard cementum. As a result, the equine head grew in size and length, stretching forward below the eyes and deepening through the jaw to accommodate the new batteries of long, large grinders and long, big incisors along with the powerful chewing muscles needed to activate them. Yet this also placed the roots of the teeth incredibly close to the floor of the sinus cavities, leaving paltry little room for deviations caused by breed type such as the crushed nasal bones of “extreme–headed” Arabians. Indeed, such heads are often typified by tooth roots actually penetrating the sinus floor, causing pain, inflammation, behavior problems, and bleeding. (However, the opposite end of the spectrum, the straight, convex, or sub–convex head, usually doesn’t have the same problems with tooth rooting or breathing when it comes to their head shape and cranial axis.)

Ultimately then, the biological result was continuously erupting teeth of about 40–42 for males and 36–40 for mares. Specifically:

12 incisors, 6 on the top and 6 on the bottom.

12 premolars, 3 on either jaw, top and bottom, part of the battery of grinding cheek teeth.

12 molars, 3 on either jaw, top and bottom, part of the battery of grinding cheek teeth.

4 canines (also called “tushes," "fighting teeth," or “tusks.”): 2 on the upper jaw and 2 on the lower jaw, erupting between the ages of 3.5–5 years always in stallions, but rarely in mares, fewer than 28%, and usually with only one or two tushes. They erupt in the diastema, and those on the top jaw are set further back than those of the bottom jaw.

Wolf teeth: An occasional small, peg–like first premolar can be present which is thought to be gradually disappearing as the horse evolves, being considered functionless. They aren’t canines as the name would imply, but vestigial premolars of Hyracotherium. Fossils show that Hyracotherium had 3 incisors, 1 canine, 4 premolars, and 3 molars on each side of the jaw. Through evolution, the second, third, and fourth premolars and the 3 molars became the long–crowned, big grinders we know today. However, the first premolar, the wolf tooth, became smaller and cone–shaped, sitting just in front of the second premolar. It’s been reported that about 40–50% of modern horses are born with up to 4 wolf teeth, most commonly only 1 or 2, which may or may not be fully erupted. Wolf teeth can be found in both stallions and mares, and usually found only on the top jaw, commonly being absent or very small in the lower jaw. They can erupt as early as birth to 6 months, but usually by 1 year. They have shallow roots and may be dislodged naturally. While wolf teeth may generally not be a problem for the horse, they’re often removed if present, being thought to interfere with the bit and so causing discomfort and behavior problems.

Adult females normally have the same complement of teeth as the males minus the canine teeth.

The incisors are used for cropping grass and the molars grind it up in preparation for the digestive process. The incisors are more curved and column–like than the molars which are more rectangular and straight (the tushes are more pointed and curved than the molars). Equine teeth aren’t pearly white, but stained with brown, yellow, orange, rust, and grey due to thicker enamel and green stains from the grasses they consume.

The teeth are also often used to determine the age of the animal with the distinct stages of growth they undergo. For example, when a foal is born, he’s generally toothless (though sometimes he’ll be born with four erupted incisors), but within the first week he’ll develop four incisors—two on top and two on the bottom of the front of the jaw, the central incisors. The second set of incisors—the intermediate incisors—erupt within the first few weeks. The final set of incisors—the corner incisors—erupt at about 6 months. Plus a few weeks after birth, the foal will also erupt 3 cheek teeth (or premolars) on each side, top and bottom, meaning six uppers and six lowers.

These temporary teeth are often called “deciduous teeth,” “caps,” “baby teeth,” or “milk teeth,” and are smoother, shorter, smaller, and whiter than permanent teeth. The incisors of milk teeth are short and square–like in shape, and also have a “neck” or “step down” at the gum line which permanent teeth lack. The corners on a 1–year–old’s incisors don’t touch either, distinct from a 2–year–old whose incisor corners do touch. The baby teeth have short roots and are designed to be popped out by the erupting permanent adult teeth.

All this means that in the first few weeks of life, a foal will erupt 16 baby teeth. Then by about 8–9 months, he’ll have a full set of 24 milk teeth—6 upper and 6 lower incisors and 6 upper and 6 lower premolars. At about this age, too, he’ll also erupt the first permanent molar set (2 on top and 2 on the bottom), erupting behind the baby premolars. By the time he’s a yearling then, he’ll erupt 24 to 30 teeth!

Then it’s during 2–3.5 years of age that largest turnover of deciduous to permanent teeth occurs. Specifically, he’ll shed 2 sets of milk incisors and 2 sets of milk premolars, all replaced by permanent teeth. Then by 3.5 years, he’ll erupt his second set of adult molars and the third set starts to erupt. That means he’ll erupt up to 24 permanent teeth in 1.5 years. Indeed, there’s a lot going on inside a horse’s mouth between 6 months and 5 years, with a full complement of baby teeth being replaced by permanent teeth plus 12 additional molars. Then at 5–6 years, most stallions and geldings erupt 4 canine teeth. That means at about 6 years, a horse will have his full set of permanent adult teeth. (Incidentally, teeth most commonly shed in the Fall.)

So because baby teeth are shed in the order they arrive, their shedding can also help determine a young horse’s age. For example, the central upper and lower incisors are the first to go at about 2.5 years; the intermediates are shed at about 3.5 years, and the corner incisors at about 4.5 years. The rate of shedding the premolars is about the same—the first set is shed at about 2.5 years, the second set at 3 years, and the final premolars at about 4 years.

Even so, because teeth are constantly growing and being worn down, the pattern of enamel wear and the shape of the tooth itself can also be used to gauge a horse’s age. For example, after five years a horse’s age can be estimated by these tooth features:

Wear on the incisor cups: A “cup” is a dark pit in the center of the tooth surrounded by enamel. It tends to disappear in the central incisors at about 5.5 years, the laterals at 6.5 years, and the corners at 7.5 years. While the cups tend to disappear first on the lower incisors, they're deeper in the upper teeth and exhibit more variation in their waning wear schedules. The vestige of the enamel surrounding the lost cup is called a “mark.”

Dental star: A secondary deposit starts to appear in the central incisors at 8–10 years, in the laterals at about 9–11 years and in the corners at 10–12 years. Stars are more flush than marks in topography and so aren’t as palpable on the tooth surface.

Shape of the incisors: The incisors change shape as the horse ages as wear takes its toll. When the horse is young (up to about 7 years), the teeth are relatively oval. They become more triangular as they erupt (about 9–13 years) to become rounder in horses older than 13 years.

Slope of the incisors: When seen from the side, a young horse’s teeth are comparatively upright, about 160˚–180˚, making his bite appear more blunt. As his teeth erupt, however, they become more sloped, decreasing to almost 90˚ in a very old horse, making them appear longer and thus the origin of the adage “long in the tooth.”

Hook: At about 7 years, a hook develops on the upper corner incisor to eventually be worn away by 9 years. In some horses, the hook reappears at about 13–14 years to disappear again.

Galvayne’s groove: Stained dark brown, a groove gradually appears at the gum line on the upper corner incisors at about 10 years, running the entire length of the tooth at 20 years to then disappear at 30 years. However, not all horses have a Galvayne’s groove.

Nonetheless, determining a horse’s age can be a bit subjective since lifestyle can alter these effects. For instance, horses raised in sandy areas usually have more wear than those raised in lush environments. Horses with bad habits like cribbing and chewing can wear their incisors down rather quickly, too.

All tooth development takes place in the maxilla and mandible, and they don’t really grow, but continuously erupt from the gums, so it’s a myth that horse teeth continuously grow for perpetuity since they do have a terminal length after which point no new tooth is produced. The working tooth surface is referred to as the “working crown” or “clinical crown,” with the remainder referred to as “reserve crown.” Of course then, this means the tooth we see as palpable in the skull isn’t the whole tooth. Because the teeth erupt progressively as more tooth is needed, their bulk is embedded in the skull to grow out and be worn down as the years pass. Indeed, the total length of the longest grinder molar in a 6–year–old horse (of about 15–17 hands) is about 7” (18cm), with only about 3/4 of its total length as root. Likewise, the total length of the longest incisor tooth is about 5.25” (13cm), with again, only about 3/4 of its length as root. Surprisingly, even the total length of a tush of that same horse, on average, is a surprising 5” (12.7cm), of which up to 2” (5cm) is root.

And how they erupt is rather interesting—it’s triggered by a tooth trying to “find” its paired mate above it. For example, the second molar on the top left side will grow until it “finds” its mate on the bottom left side. Yet the rate at which equine teeth erupt is based on individual genetics and metabolism which can be influenced by management and environmental conditions. Nevertheless, the basic rate of tooth eruption is about 1/3 inch (.84cm) a year. Given that the horse has about 6” (15cm) of molar crown, this means the tooth will be functional for about 20 years.

What’s also particularly important for artists to notice is how tooth eruption over time affects a horse’s head shape. If we study closely, we’ll see that a horse’s head often starts out more wedge–shaped because of all the long, new teeth, often with hefty bars. “Tooth bumps” (also called “eruption cysts”) can occur in horses 2–5 years old, manifesting as lumps along the jaw bars. They’re caused by the pressure of the permanent molars (especially premolars 3 and 4), which are still under the gum line, trying to push the baby teeth out. However, as the teeth are worn down with age, the head usually becomes progressively more rectangular, becoming shallower through the bars as they thin and “collapse” around the shortening tooth roots. This is one of the reasons why the heads of senior citizens look quite different, being a bit more rectangular than the heads of younger horses.

The equine tooth itself is typified by a layering of materials of differing hardness which results in different rates of wear. Enamel is hardest and so wears the slowest, creating sharp ridges as the cementum and dentin are worn away faster. As the horse chews then, his teeth wear like self–sharpening knives, always maintaining the necessary edge to pulverize grasses effectively. And the more wear that happens, the sharper the horse’s teeth become. That’s important because parts of the silica skeleton not smashed won’t digest since even gut bacteria cannot penetrate the silica shell. Altogether, this layered structure can withstand the abrasive silica particles to efficiently turn grasses into the necessary consistency of fine, milled cornmeal to increase digestive efficiency and protect against intestinal blockage.

For these reasons, it’s no small statement to say a horse is only as healthy as his teeth. Indeed, his lifespan is essentially decided by his teeth—how long can he grind his food into the critical consistency of fine cornmeal? In this way, his teeth are important not only for efficient digestion, but for his very life! The fine milled–cornmeal texture produced by his teeth is necessary to avoid lethal blockages in his digestive system since it’s vulnerable to obstruction through its various bottlenecks. For example, colic is a typical outcome of an intestinal blockage. In particular, any grass bits longer than 3/8–1/2” (especially 1/2–1”) can be lethal to a horse by causing such an obstruction. The horse doesn’t chew his cud nor can he vomit so whatever he swallows has a one–way trip. In this way, equine teeth not only make food digestible, but also safe to eat. Quite a big responsibility for such unassuming anatomy!

Interestingly, when he becomes incapable of pulverizing grass into the texture of fine cornmeal, he’ll begin to starve which is one of the sad reasons why seniors can become so thin and frail—it’s not because of their age per se, it’s because of their worn–down, spent teeth. In response, old horses often “quid,” or chew a wad of grass to extract the juices to then spit out the fibrous wad. They know the danger of improperly ground foodstuffs that could cause deadly blockages in their digestive tract. However, owners can intervene by providing fine or “pre-chewed” mashes to extend the life of their senior equine friends.

Being so, perhaps one of the most improved areas of equine health care involves dental procedures. Research has shown that when teeth are properly cared for, the horse can masticate his food better and, as a result, make safer and better use of the nutrients being ingested—and that translates into a longer, happier life.

Yet his teeth also had to change orientation in his skull as well. In particular, they had to form into front incisors for nipping off grass when the head was in the grazing position and then large grinders in the back of his jaw to smash it. This telescoped the skull from below the eyes while also creating a long diastema, the space between the front incisors and the back molars (where the bit sits). For example, about 40 million years ago, the head of Mesohippus began to elongate, having a battery of grinding teeth increased to six (as his premolars adapted into grinders) on either side of his jaw, top and bottom—the same number in living horses today.

Over time then, the head of Equus would become longer and larger, pulling the teeth out from under his eye orbits and deepening his jaws to make room for these new, bigger, high–crowned teeth accompanied by the large, powerful chewing muscles required to activate them. Also a benefit of this new, long head was the placement of his eyes relatively “higher” in relation to ground level, allowing him to scan the grass line for predators when in the head–down grazing position. In other words, his lips would be at ground level for nipping off grasses while his eyes were higher up to watch for threats.

Furthermore, there’s no joint at the mouth, around the chin. Instead, the mechanics of the skull dictate that the jaw opens at the joint between the mandible and the zygomatics, behind the eye, dropping the entire lower jaw when the mouth is opened. The biomechanics of his mandible joint also changed away from for and aft motion to instead facilitate side–to–side rotary or triangular motion, or shearing action, something still distinctive today. And because the horse’s lower jaw is narrower than the upper jaw, his upper teeth are offset and overhang the lower teeth, or in other words, the lower teeth are positioned to the inside of the upper teeth.

His lips also adapted to grazing, which in the modern horse have an almost prehensile quality and are unusually sensitive and adept at selecting choice bits of food. Indeed, his lips are used for exploration of new objects and, in a way, can be likened to his “hand.” This design was produced by a shortening of his nasal bones to permit a fleshier and more flexible muzzle advantageous for selective grazing. Fortuitously, the shortened nasal bones also allowed his nostrils to enlarge and become fleshier, enabling greater intakes of air for sustained galloping.

Conclusion To Part 2

It’s an interesting evolutionary story, isn’t it? And we're just starting! As equine artists, we can get caught up in the structure of muscles and bones and—sure—we can get by with the whats. But if we neglect the whys for those muscles and bones, we'll have missed what it means to be equine in the first place. And it's that evolutionary backdrop which gifts us with the perspective needed to better acknowledge his full narrative, authentically and responsibly. Truly, there's a big difference between knowing structure and understanding it. To be an insightful equine artist then means we first need to be equine biologists. Every feature of his physiology has a story to tell, one that can enrich our depictions in untold ways. So to overlook his evolutionary biology is to miss the world from his perspective, yet isn't this one of the overarching stories we want to tell? If all we do is focus on the whats of anatomy, won't we have missed something far more important? Equine anatomy is much more than skin deep! Far bigger than mere structure! Ultimately, we're really telling the story of what it means to be equine, in the full depth of what that means. To consider this dimension then is to provide an opportunity for our grasp of structure to grow in sophistication and scope, and in ways that lend substance to our portrayals. As such, our work evolves beyond prosaic representation to become something that transcends this failing of realism to embody something more—equineness itself. In doing so, our work comes to speak for this animal in ways otherwise impossible, and in ways that not only imbue that elusive élan vital, but also offer an ode to viability often missed by those who lack such context.

So—no—we’re not done yet! There’s a lot more to explore as his body changed to adapt to his new niche, a whole host of modifications that speak to his biological truth. In Part 3 then, we’ll continue our discussion with his eyes, an important feature for the species, in more ways than one. So until next time…chew on all this tasty information!

“The trick is to keep exploring and not bail out, even when we find out that something is not what we thought.” ~ Pema Chodron